Sensors for trans-illuminating human tissue, or for recording illumination reflected by human tissue, in particular for pulse oximetry, have to fulfill several demands. The most important one is surely that the sensor has to provide a high quality physiological signal, in order to produce accurate readings. Thus, good skin contact, as well as immunity against patient movement, is required. The sensor should also provide sufficient comfort for the patient, i.e., it should not impair his well-being. Last, but not least, easy handling by medical personnel has to be ensured.
Apart from these physiological and medical considerations, the sensor should be easy to manufacture, at reasonable costs.
There have been several attempts in the prior art to fulfill the above needs. DE-A-37 03 458 discloses an elastic, self-contained sensor for application to a human finger. Another solution, also for application to a human finger, is discussed in U.S. Pat. No. 4,685,464. The latter document describes a sensor of the clothes-peg type.
There have also been prior art sensors which could be secured to the human skin by an adhesive. Such a sensor is disclosed in U.S. Pat. No. 4,830,014.
One will note that all of the above medical sensors are designed for application to a specific limb or other part of the human body. So far, the prior art sensors have revealed acceptable results.
However, one serious disadvantage of the prior art sensors (in particular, in terms of cost) is the above discussed aspect that different sensors have to be provided for different sites of application. The variety of different sensors necessitates intensive store-keeping, as a multiplicity of sensors have to be taken on stock. Another aspect is that different sensors require different product runs and are thus not as cheap to manufacture as would be the case if only a single sensor, with the associated number of pieces, would be manufactured. There are also clinical aspects which would make it desirable to have a single sensor only, instead of a variety of different sensors. For example, a patient may be monitored with a foot sensor, but it is desired, in the course of medical treatment, to continue monitoring with an ear sensor. In such a case, the foot sensor has to be completely removed and disconnected from the monitor; similarly, the ear sensor has to be applied, fastened, and connected to the monitor. Alarms have to be disabled during this process in order not to generate an alarm when the foot sensor is removed.
Another clinical problem in this context is that medical personnel have to be trained to use and apply all different kinds of sensors correctly. This is of particular importance in order not to miss any dangerous condition, e.g., a stress situation, of a patient.
The underlying problem becomes even more apparent in respect of disposable sensors, such as adhesive ones. If a patient is monitored at various different locations of the body, several sensors will have to be used, which are all thrown away afterwards.
It would therefore be desirable to have one common sensor for all of the various places of application. However, the prior art could not provide any solution. Therefore, as the sensors have necessarily to be adapted to the shape or geometry of the human skin at the place of application, in order to ensure tight contact with the skin. For example, a self-contained sensor of the type disclosed in DE-A-37 03 458 (which is intended for application to a human finger) cannot be applied to the human ear, or the human foot. Likewise, a sensor of the clothes-peg type (U.S. Pat. No. 4,685,464) cannot practically be applied to a foot (unless it is applied to a toe which is not a preferred location). Similar considerations apply when comparing sensors intended for use with an adult person to sensors intended for neonates.